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Atlas of Clinical Imaging and Anatomy of the Equine Head
Larry Kimberlin, Alex zur Linden, Lynn Ruoff
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eBook - ePub
Atlas of Clinical Imaging and Anatomy of the Equine Head
Larry Kimberlin, Alex zur Linden, Lynn Ruoff
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Atlas of Clinical Imaging and Anatomy of the Equine Head presents a clear and complete view of the complex anatomy of the equine head using cross-sectional imaging.
- Provides a comprehensive comparative atlas to structures of the equine head
- Pairs gross anatomy with radiographs, CT, and MRI images
- Presents an image-based reference for understanding anatomy and pathology
- Covers radiography, computed tomography, and magnetic resonance imaging
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CHAPTER 1
Overview of CT and MRI of the Equine Head
The diagnosis of conditions affecting the equine head is challenging for the veterinary practitioner due to its large size, complex anatomy, and the multitude of different tissues present and thus the large number of potential disease processes. Disease processes of the teeth include caries, periodontal disease, tooth root abscess, tooth fracture, dentigerous cysts, and malocclusion to name a few [1]. The tongue can be affected by trauma, infection, or neoplasia. The nasal passages and paranasal sinuses are important parts of the equine head that can be the site of sinusitis, ethmoid hematomas, cysts, or neoplasia. The diverticulum of the auditory tube (guttural pouch) can develop fungal granulomas, empyema, blood clots, or tympany. Laryngeal hemiplegia, dorsal displacement of the soft palate, epiglottic entrapment, rostral displacement of the palatopharyngeal arches, arytenoid chondritis, and pharyngeal narrowing can affect the pharyngeal region [2]. Other tissues in the head such as the lymph nodes and salivary glands can be affected by infectious or nonâinfectious inflammation, or neoplasia. The brain can be affected by trauma, bleeding, infarction, neoplasms, cholesterinic granulomas, ventriculomegaly (hydrocephalus), and infection (meningitis or meningoencephalitis). Trauma to the head can result in fractures of the calvarium, mandible, temporomandibular joint, basisphenoid bone, and nuchal crest of the occipital bone. Although uncommon in horses, neoplasia that can be found in the head includes melanoma, adenocarcinoma or rhabdomyosarcoma of the tongue, lacrimal gland adenocarcinoma and ophthalmic tumors associated with the eye, and multicentric lymphoma affecting the lymph nodes in the head [2]. The hyoid bones can be affected by fractures or temporohyoid osteoarthropathy. The eyes and ears are also prone to a variety of pathological conditions. Many of these conditions can be diagnosed on physical examination; however, many require further diagnostics.
Diagnostic imaging of the equine head is most commonly done via radiography (Figure 1) or endoscopy. Routine radiographic examination can include orthogonal projections of the area of interest, oblique projections of the dental arcades or temporomandibular joints and intraoral projections for the rostral mandible/maxilla. Due to the size of the adult head and the limited size of the Xâray cassette or imaging plate, multiple radiographs are needed to image the entire head, although this is not routinely performed in clinical practice. Radiographs offer superior spatial resolution compared to more advanced imaging options; however, due to the superimposition of anatomy, lesion localization can be quite challenging using radiography. The anatomy of the head is complex and radiographs do not provide adequate contrast of the soft tissues of the head. Radiographic anatomy has been thoroughly described elsewhere and is outside the scope of this book.
A computed tomography (CT) unit consists of a highâpowered Xâray tube mounted in a circular gantry across from a detector array. The gantry is able to rotate around the patient using slipâring technology so it is not tethered electronically to the rest of the unit. As the gantry rotates, the patient moves either into or out of the gantry as the Xârays are absorbed, scattered, or pass through the patient. The Xârays that reach the detector array are used to construct an image.
For digital radiographs, the attenuation of the Xârays results in a twoâdimensional image involving multiple pixels. CT uses a similar method to display an image by converting a volume of tissue to a threeâdimensional pixel called a voxel. CT will determine the average linear attenuation coefficient of Xârays for each voxel in a patient at a particular location [3]. Each voxel can be given a quantifiable number in terms of its gray scale, termed a Hounsfield unit (HU). As a reference, pure water has a HU of 0 and air is â1000 HU. Adipose tissue can vary from â30 to â80 HU, soft tissues +30 to +220, while bone and iodinated contrast media can be close to +2000 to +3000. Each voxel is then interpreted as a pixel when displayed as a twoâdimensional CT image.
Most CT images are reconstructed in an axial plane. If the depth of the slice thickness (zâdirection of the voxel) is the same as the size of the pixel (x and y directions), then the voxel is considered isotropic, or near isotropic if it is similar in size. Isotropic voxels allow for highâresolution reconstructions of the CT dataset into multiple different planes. These reconstructions allow one to view the anatomy in different planes to identify the extent of a disease process or to better visualize the âthreeâdimensionalâ (3D) anatomy using a twoâdimensional interface. Isotropic voxels can also be used to produce highâresolution reconstructions that appear threeâdimensional, even though they are still a twoâdimensional image. This is demonstrated in Figure 2c and d, where a 3D reconstruction can be useful to get an overall look at the scanned anatomical structures.
Computed tomography images are reconstructed from a very large collection of voxels (raw image data), each with its own Hounsfield unit and location in space. The computer uses an algorithm or filter to adjust how each pixel looks on a twoâdimensional image and this algorithm can be modified to alter the spatial resolution and contrast differences of different tissues. The primary two algorithms used in this book are referred to generically as a bone filter or algorithm and a soft tissue filter or algorithm. The bone algorithm has a higher spatial resolution and the bone and teeth are seen in gray with wellâdefined edges, whereas all the soft tissues are homogenously gray. The soft tissue algorithm has a reduced spatial resolution but the contrast of the soft tissues is more noticeable, and the bone and teeth are completely white.
The appearance of the CT images can be adjusted by the viewer for either algorithm using the window width/window level adjustment function found on all image viewing systems. The window width (WW) is the range of displayed Hounsfield units. The window level (WL) is the Hounsfield unit in the center of the window width. In this book, the bone algorithm is shown in a bone window, with a WW of ~3500 and a WL of ~650. The soft tissue algorithm is shown in a soft tissue window, with a WW of ~500 and a WL of ~70. So, for the bone setting a wide window of 3500 densities allows for a lot of different densities to be displayed, centered on 650 HU (+4150 to â2850 HU). For the soft tissue setting, a short window width of 500 is used, centered at 70 HU, near the density of the soft ti...
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Normes de citation pour Atlas of Clinical Imaging and Anatomy of the Equine Head
APA 6 Citation
Kimberlin, L., Linden, A. zur, & Ruoff, L. (2016). Atlas of Clinical Imaging and Anatomy of the Equine Head (1st ed.). Wiley. Retrieved from https://www.perlego.com/book/991818/atlas-of-clinical-imaging-and-anatomy-of-the-equine-head-pdf (Original work published 2016)
Chicago Citation
Kimberlin, Larry, Alex zur Linden, and Lynn Ruoff. (2016) 2016. Atlas of Clinical Imaging and Anatomy of the Equine Head. 1st ed. Wiley. https://www.perlego.com/book/991818/atlas-of-clinical-imaging-and-anatomy-of-the-equine-head-pdf.
Harvard Citation
Kimberlin, L., Linden, A. zur and Ruoff, L. (2016) Atlas of Clinical Imaging and Anatomy of the Equine Head. 1st edn. Wiley. Available at: https://www.perlego.com/book/991818/atlas-of-clinical-imaging-and-anatomy-of-the-equine-head-pdf (Accessed: 14 October 2022).
MLA 7 Citation
Kimberlin, Larry, Alex zur Linden, and Lynn Ruoff. Atlas of Clinical Imaging and Anatomy of the Equine Head. 1st ed. Wiley, 2016. Web. 14 Oct. 2022.